1. Field of the Invention
The present invention relates to an image recording method and an image recording apparatus, and more particularly, to image correction technology relating to defective recording elements in a recording head having a plurality of recording elements.
2. Description of the Related Art
An inkjet recording apparatus, which forms a desired image on a recording medium by an inkjet method, is an image recording apparatus widely used. The inkjet recording apparatus can include a full-line inkjet head having nozzles (recording elements) arranged in a two-dimensional configuration and achieve high productivity by means of single-pass image recording, which forms an image on the whole surface of paper by performing just one scanning action of the full-line head and the paper.
In an inkjet recording apparatus, a defective recording element which does not have a normal ink ejection state gives rise to various problems. In particular, in the single-pass image recording using a full-line head, stripe-shaped density non-uniformities or banding occur in the recorded image as a result of error in the recording characteristics, such as recording position error, ejected droplet error, ejection failure, and the like, which are caused by defective recording elements, and hence the image quality declines dramatically. Various technologies have been proposed in order to avoid such density non-uniformities caused by the occurrence of defective recording elements.
For example, a method is used in which defective recording elements are disabled for ejection (masked), and a pixel corresponding to each defective recording element is compensated by using pixels recorded by normal recording elements which are peripheral to the defective recording element. Possible examples are technology which corrects density non-uniformities by adjusting the output characteristics of the recording elements in accordance with the characteristics of individual recording elements, and technology which carries out density non-uniformity correction by changing the image information data (tone values) of the pixels assigned to the recording elements. In the former technology, the actual ejection characteristics, such as the droplet volumes, are changed by altering the ejection drive conditions for the respective recording elements, thereby adjusting the dot diameters and the dot density. Implementing this technology places restrictions on the method used in the head and the range of correction. On the other hand, a possible example of the latter technology is the image recording method described in Japanese Patent Application Publication No. 2007-160748. The method which changes the image information data of the pixels which are recorded about the periphery of the pixel to be recorded by the defective recording element, has a high range of freedom and therefore highly accurate correction can be expected.
In the image recording method described in Japanese Patent Application Publication No. 2007-160748, information containing recording characteristics including the recording position errors and the ejected droplet volume errors of the recording elements is acquired, N compensation recording elements (where N is an integer larger than 1) for use in correction of output density are set, and density correction coefficients are specified for the N compensation recording elements according to correction conditions including conditions where a differential coefficient at a frequency origin point in a power spectrum representing spatial frequency characteristics of the density non-uniformity becomes substantially zero.
In the related art, the compensation for defective recording elements is performed by changing image information data of pixels which are peripheral to the pixel to be recorded by each defective recording element, and the concept is of correcting density error caused by the defective recording element by using a plurality of normal recording elements which are positioned at the periphery of the defective recording element.
FIGS. 12A to 12D are schematic drawings for describing the image correction method to compensate defective recording elements in the related art. A recording head 300 in FIG. 12A has a structure in which a plurality of recording elements 302 are arranged in one row at equidistant intervals in a direction (the horizontal direction in FIG. 12A) which is substantially perpendicular to the recording medium conveyance direction (the vertical direction in FIG. 12A), and these recording elements include a defective recording element (a recording element which is not capable of recording desired dots) 304. FIG. 12B shows an image 312 formed on a recording medium 310 by a single-pass method, using the head 300 having the defective recording element 304. Stripe-shaped density non-uniformity 314 following the recording medium conveyance direction occurs in the image 312 at a position corresponding to the defective recording element 304.
FIG. 12C is a diagram showing set values of recording elements in image correction to compensate the defective recording element 304 (see FIG. 12A). When P is taken to be the set value corresponding to the density that has been originally supposed to be recorded by the defective recording element 304, then the set values of the normal recording elements 306A and 306B which are adjacent to the defective recording element 304 on either side thereof in order to correct stripe-shaped non-uniformity occurring due to the presence of the defective recording element 304 is P×M.
FIG. 12D shows a schematic drawing of a density distribution obtained from a macroscopic view of the density of a two-dimensionally recorded image after conversion to one dimension. As shown in FIG. 12D, the density 320 before the correction has low density at the position corresponding to the defective recording element 304, but the density 322 after the correction maintains uniform density throughout, without any reduction in the density at the position corresponding to the defective recording element 304.
In a recording head in which a large number of recording elements are arranged in a two-dimensional configuration, it is readily envisaged that defects may occur in a plurality of recording elements. Moreover, there are cases where the plurality of defective recording elements are in close proximity to each other to the extent of affecting the appearance of recorded image as observed by the human eye. In cases where a plurality of recording elements in mutual proximity have suffered abnormalities, if the above-described compensation for an isolated defective recording element is directly applied, then insufficient compensative functions are displayed and it may be impossible to obtain desired effects.
FIGS. 13A to 13D are illustrative diagrams of a case where the compensation for an isolated defective recording element is directly applied to the compensation for a plurality of defective recording elements which are present in adjacent positions. As shown in FIG. 13A, a case is considered in which two defective recording elements 304-1 and 304-2 are present at positions which are spaced by three elements apart. When image recording is carried out in a single-pass method using this head 300, then as shown in FIG. 13B, stripe-shaped density non-uniformities 314 occur in the recorded image 312 at the positions corresponding respectively to the two defective recording elements 304-1 and 304-2.
Then, as shown in FIG. 13C, when the set value of the recording elements 306A, 306B, 308A and 308B which are adjacent on either side of the two defective elements 304-1 and 304-2 is P×M, similarly to the case where there is one defective recording element (i.e., where there is an isolated defective recording element), then as denoted with reference numeral 332 in FIG. 13D, it can be seen that a problem occurs in that a dark line (the mounded portion 334 represented with a broken line in FIG. 13D) is visible in the recorded image due to the close proximity of the two recording elements 306B and 308B which have increased set values for the purpose of compensation.
In the technology described in Japanese Patent Application Publication No. 2007-160748, if there are two or more ejection failures in the correction range, then stripe-shaped non-uniformities are judged to be of a level that cannot be corrected, and the method transfers to a head cleaning mode. Furthermore, Japanese Patent Application Publication No. 2007-160748 describes excluding ejection failure nozzles from compensative nozzles if mutually adjacent nozzles are suffering ejection failure, but makes no mention of specific compensation performed in a case where a plurality of mutually adjacent nozzles are suffering ejection failure.